appending to a 2D list in python where the row/col length is unknown

Question

I am looking to flip the rows and columns on a 2D list and am running into trouble. Python won't allow a value to be assigned to an uninitialized 'cell'. I am not sure how to initialize my grid since I don't know the length ahead of time. The following:

data = list()
maReader = csv.reader(open('TEST.csv', 'rb'), delimiter=',', quotechar='"')
for rindex, row in enumerate(maReader):
    for iindex, item in enumerate(row):
        data[iindex][rindex] = item

fails with a "IndexError: list assignment index out of range" error. I could loop through twice, the first time to count columns and rows, than initialize the grid, than loop again and assign values, but that seems awfully wasteful.

I am pretty new to Python and figure there is a simple way to do this that I am missing.

Thanks for the help.

score 3 · Accepted Answer · answered Oct 25 '11 at 21:47

3

Here's an EAFP counterpoint to @retracile's LBYL approach.

data = [] #list()
maReader = csv.reader(open('TEST.csv', 'rb'), delimiter=',', quotechar='"')
for rindex, row in enumerate(maReader):
    for iindex, item in enumerate(row):
        try:
            data[iindex].append(item)
        except IndexError:
            data.append([item])

answered Oct 25 '11 at 21:47

Marty

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Since the first row is the only one to hit the exception, this approach is probably faster than mine. – retracile Oct 25 '11 at 21:51

score 2 · Answer 2 · answered Oct 25 '11 at 21:22

Assuming all rows of equal length, you can approach it like this:

data = list()
maReader = csv.reader(open('TEST.csv', 'rb'), delimiter=',', quotechar='"')
for rindex, row in enumerate(maReader):
    for iindex, item in enumerate(row):
        if len(data) <= iindex:
            data.append([])
        data[iindex].append(item)

score 2 · Answer 3 · answered Oct 25 '11 at 21:45

2

This should do:

maReader = csv.reader(open('TEST.csv', 'rb'), delimiter=',', quotechar='"')
data = zip(*maReader)

Though, zip gives a list of tuples. If you need a list of lists you can do either of these:

data = [list(x) for x in zip(*maReader)]
data = map(list, zip(*maReader))

answered Oct 25 '11 at 21:45

Avaris

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I _knew_ there was a better way, but my brain just wasn't working. +1 – retracile Oct 25 '11 at 21:52

score 1 · Answer 4 · edited May 23 '17 at 11:46

1

if all rows of equal length then data = zip(*maReader) should do it. If not:

from itertools import izip_longest

list(izip_longest(*f, fillvalue=''))

appending to a 2D list in python where the row/col length is unknown

4 Answers4